Photodiodes are important in many areas of technology. They are essential components of optical communication systems. Such a system essentially comprises a modulated light source coupled to a photodiode by an optical transmission fiber. The diode converts the transmitted optical signal into a corresponding electrical signal.
In optical communication systems it is desirable to have highly sensitive photodiodes. Sensitive photodiodes reduce the number and frequency of repeaters. Sensitive photodiodes are also important in other applications such as time domain reflectometers.
Photodiodes are essentially reverse-biased semiconductor pn junctions. The reverse bias creates an enlarged depletion region between the n region and the p region. Light of appropriate wavelength entering the depletion region generates electron-hole pairs. The electrons and holes are separated by the reverse-bias voltage and produce a current proportioned to the amount of incident light. Photodiodes of different materials are responsive to different wavelengths.
This simple diode structure can be enhanced for greater sensitivity. One improvement is to substantially increase the electrical field across the pn junction. An ordinary photodiode produces but one electron-hole pair for each photon of light absorbed. But if the electrical field is substantially increased, it will impart sufficient energy to the separated electrons and holes that they, upon collision with neutral atoms, will generate additional electron-hole pairs. This process creates an avalanche of charge carriers, increasing the output current. Such diodes are called avalanche photodiodes or APDs.
Several adaptations make APDs more useful in optical communications. Present optical communications systems typically operate at wavelengths in the regions of 1.3 and 1.5 microns. To detect these wavelengths, APDs are commonly made of III-V semiconductors such as indium gallium arsenide and indium phosphide. Such semiconductors have a comparatively small band gap with the consequence that the high fields required for charge carrier multiplication can produce high dark currents due to quantum mechanical tunneling (noise). This problem is alleviated by providing separate absorption and multiplication regions. The carrier pairs are photogenerated in one absorbing region and then swept into a wider bandgap layer containing the pn junction where avalanche multiplication takes place.
An important class of high performance APDs is made from bonded crystalline substrates. Such devices, referred to as heterointerface photodiodes, use one crystalline material as an absorbing region and another crystalline material for multiplication. The carrier pairs are swept through the heterointerface between the two crystals. Such devices are described in A.R. Hawkins et aL., "Silicon heterointerface photodetector," 68 Appl. Phys. Lett., pp. 3692-94 (1996), which is incorporated herein by reference.
The quality of such devices depends in large measure on the quality of the heterointerface between the two bonded crystals. A method of bonding crystals to produce high quality heterointerfaces is described in the copending U.S. patent application Ser. No. 09/369,682 filed by applicant and others on Aug. 5, 1999 and entitled "Method For Bonding Two Crystalline Substrates Together." This application is incorporated herein by reference.
Applicant has further discovered that the quality of the interface and the device also depends on other modifications of conventional processes.